Unoccupied electronic structure of Ru(0001)

The unoccupied electronic states of the clean Ru(0001) surface were measured along the Gamma K and Gamma M high symmetry axes of the surface Brillouin zone (SBZ) using angle-resolved inverse photoemission spectroscopy. The spectra are dominated by strong emission within the first 2 eV above the Fermi level and, but for a weak image potential state, are essentially featureless at higher energies (up to similar to 6 eV). The spectra obtained along the Gamma K direction exhibit three well-defined features that are identified as originating from bulk direct transitions. The dispersions with parallel momentum for two of the three features run along the edges of projected band gaps found in ab initio electronic structure calculations. Detailed examination of these spectra reveals the presence of a surface state that appears near k(parallel to)=0.7 angstrom and persists to the boundary of the SBZ. The spectra along the Gamma M direction exhibit less well-defined features, but comparing spectra from the clean surface to those obtained from a surface exposed to 0.5 L (1 L=10(-6) Torr s) of CO enables identification of four additional peaks arising from bulk direct transitions. One feature may be associated with a surface resonance that is predicted to exist near the M point. Comparison of the observed dispersions with those predicted for states along the high symmetry directions of the bulk gives a good account of the experimental data.